The present invention relates to ultrasound transmitters and methods for transmission that suppress sidelobe levels in coded excitation ultrasound imaging. Sidelobes cause imaging artifacts. The sidelobes are spatially offset energy away from the desired location. This would create artifacts around the desired imaging targets.
Coded excitations have been used to reduce or cancel sidelobes. High bandwith transmitters capable for long time transmissions are used for coded excitation waveforms, such as chirp, Baker, Golay etc. coded waveforms. A combination of one or more of phasing, amplitude and frequency changes are coded into a transmit pulse. On receive, a filter applies a more or less modified inverse of the coding to isolate desired information, canceling or reducing sidelobe information. For example, in Golay coding, two sequential transmissions have orthogonal phases after receive processing. The responsive receive sets of data are auto-correlated or convolved. The addition of convolved sequences cancels or reduces the oppositely phased sidelobe levels. However, the Golay coding uses two transmissions for sidelobe suppression, resulting in halving the frame rate. As another example, linear or sinusoidal waveforms with chirp coding are generated. A single transmit chirp coded excitation provides axial sidelobe suppression with an improved frame rate as compared to the Golay coding example. However, the transmitters or waveform generators used to form linear chirp signals are complex and expensive.
Another method of sidelobe suppression is transmission of rectangular waveforms. Unipolar and bipolar ultrasound transmitters generate energy at harmonic frequencies of the desired fundamental transmit frequency. Harmonic energy may result in increased sidelobe levels on the receive side. U.S. Pat. No. 5,833,614 discloses using pulse width modulation to suppress the harmonic content of transmitted bipolar and unipolar waveforms. The width of the unipolar and bipolar pulses within a transmit waveform are varied as a function of a desired window.